In the regeneration of spent catalyst, the burning of the coke elevates the temperature of the catalyst. Modern zeolite catalysts rapidly lose activity at temperatures of about 1500.degree. F. which imposes a temperature limit on the regenerator. Normally, a regenerator accumulates the particulate catalyst in a fluid bed. While the average temperature through the bed may be below the critical deactivation temperature, it is also necessary that the particle temperature not exceed the temperature level at which deactivation occurs. There can easily occur local, minute hot spots in which some catalyst is deactivated. This is highly undesirable and should be avoided if possible.
The temperature of a particle of catalyst, as opposed to that of the bed as a whole, is a function of the supply of oxygen as indicated by its partial pressure, the availability and concentration of coke and other volatile hydrocarbons which are the fuels consumed by combustion, the degree of mixing of heavily and lightly coked catalyst particles in the catalyst bed and the distribution of coke on the catalyst particles.
The present invention contemplates introduction of newly supplied spent catalyst to a dense phase fluid bed where the freshly introduced catalyst is distributed steadily and evenly over the top side of the catalyst bed. It is distributed and dispersed over or in the upper regions of the bed separated from the place of introduction of air such that a substantial portion of the oxygen has partially been depleted in combustion of coke in the lower regions of the bed. This enables the newly introduced heavily coked particles to achieve initial burning with a controlled increase in temperature at the top of the catalyst bed.
The present invention is directed to a method and apparatus which is able to consume more of the coke during regeneration at the most expedient time, thereby achieving an opportune disposal of the coke without creating hot spots and the consequential catalyst deactivation normally resulting from hot spots. The particles of catalyst in the lower portions of the bed have substantially less coke thereon but are exposed to more oxygen. This also provides a uniform and regionally increased temperature which tends to elevate the temperature of the bed as a whole but reduces the temperature differential occurring between the bed as a whole and the individual particles. The catalyst which is removed from the center lower portions of the bed achieves a higher temperature, thereby reducing the amount of pre-heat required for the feed stock which saves utilities.
Another advance provided by the apparatus is the combustion of the coke in the dense phase fluid bed. The regeneration of spent catalyst involves three main reactions: EQU CO.sub.2 +.dbd. .dbd.2CO EQU 2co+o.sub.2 .dbd. .dbd.2co.sub.2 EQU c+o.sub.2 .dbd. .dbd.co.sub.2
the last equation describes the net effect of the other two oxidation reactions, and is the most desirable. To the extent that any carbon monoxide is liberated, the second equation is desirable. When carbon monoxide is disposed of a flue gas it is, in effect, a wasted fuel but its combustion in the fluid bed can provide the heat necessary to raise the regenerator temperature significantly. However, if the combustion of the carbon monoxide occurs in the upper portions of the regenerator chamber, it will heat the flue gas leaving the regenerator. It is advantageous for it to be combusted in the fluid bed, rather than afterburning above the surface of the bed. The present invention is constructed and arranged so that a substantially high percentage of any carbon monoxide liberated is combusted within the bed to thereby heat the circulating catalyst.